The present invention relates generally to methods and devices for monitoring chatter in twin drives for roll stands, and more particularly to a method and device for monitoring chatter in twin drives of roll stands in which the two rolls are driven by separate motors.
In a twin drive for a roll stand, the upper roll and the lower roll of the roll stand are driven separately by an upper motor and a lower motor, respectively. The coupling of each motor through a more or less torsion-proof shaft to the corresponding roll produces a structure capable of rotational oscillation. Pronounced changes in the frictional values in the roll nip can trigger a self-excitation process, in which the rotational energy supplied to the rolls is converted into torsional oscillating energy. The mechanical rotational oscillation behavior of the twin drive is thus undamped in such a way that the rotational speed regulation for the motors can no longer provide sufficient stabilization. If the adhesion between the two rolls is lost, slipping processes occur and as a consequence thereof, so-called chatter occurs.
It is known that chatter can be detected by measuring the oscillations of the drives and determining if a predetermined amplitude is exceeded by these oscillations. In the event an amplitude of the oscillations is exceeded, a chatter detection signal is generated that is delivered to the rotational speed regulator to reduce the rotational speed until the chatter stops. Operational vibrations triggered by a pass in the roll stand must not, however, result in a response of the chatter monitoring system. In the past, well-damped pass vibrations could only be selected through the level of the amplitude threshold of building chatter vibrations. Often, however, the time remaining for a reaction is too short, so that damage to the drive can occur.
Hence, the present invention is directed to the problem of developing a method for permitting faster and more reliable differentiation of operational pass vibrations and chatter vibrations. In addition, the present invention is directed to the problem of developing a device for implementing the method of the present invention.